This invention is related in general to the conversion of kinetic wind energy into rotary shaft power. More particularly, it concerns the structural problems of vertical axis windmills.
The advantage of a structurally stable vertical axis windmill, as compared, for example, to the standard horizontal axis windmill, is that no steering mechanism sensitive to wind direction is needed, and therefore, no gearbox at the top of the supporting tower is needed.
Previous designs of vertical axis windmills generally fall into one of three categories, i.e., the giromill, the Savonius rotor, and the Darrieus rotor. The giromill is a carousel-shaped structure having vanes which automatically orient themselves in favorable relations with respect to the wind direction. The Savonius rotor is essentially a pair of opposing half-cylinders, which are offset from the vertical supporting axis in an S-shaped configuration, so as to utilize the difference between the wind force acting against a convex surface and the wind force acting against a concave surface. The giromill and the Savonius rotor are inherently inefficient because of their necessarily slow rotational speeds.
The conventional Darrieus rotor is essentially a thin hoop whose diameter constitutes a vertical axis of rotation. The cross-section area of the long continuous ribbon forming the hoop has the profile of a symmetrically streamlined airfoil. For each of the two half-hoops attached to the vertical axis of rotation, the blunt leading edge of the airfoil-shaped ribbon is oriented so as to head into the relative airflow caused by rotation of the hoop about its verticalized diameter.
The airfoil chord length, i.e. the ribbon width, is typically equal to about 10 percent of the hoop radius, and the airfoil thickness is typically equal to about 12 percent of the chord. Because the hoop is not self-starting, an auxiliary power source is required to bring the rotor up to operational speed. Optimum power is extracted from the wind when the tangential velocity of the outermost sectors of the hoop is typically 4 to 6 times the ambient wind speed. The energy conversion efficiency of this type of windmill is theoretically expected to be in the range of 30 to 40 percent.
The most difficult problem encountered in implementing the conventional Darrieus rotor concept stems from its requirement of a vertical axis which is tall, slender, and rigid. One approach to this problem has been to extend the required vertical shaft upward from a pair of bearings located in the top and bottom of a heavy gearbox mounted on a sturdy ground base. Such a configuration puts the center of mass excessively high above the center of support. Another approach has been to make the hoop supporting shaft hollow so as to fit, with the aid of an upper-end bearing and a lower-end bearing, over a vertical flagpole-type shaft which extends upward from a sturdy ground slab to a high top plate from which guy wires slant downward and outward far enough to clear the hoop. Such a configuration introduces a noxious visual appearance and has the disadvantage of requiring an excessively large base to accommodate the guy-wire anchoring points.